This invention relates generally to the production of chlorine dioxide. More particularly the present invention relates to the method of fabricating a coated electrode and the particular anode structure used in the electrochemical process to manufacture chlorine-free chlorine dioxide from dilute alkali metal chlorite solutions in an electrolytic cell. Chlorine dioxide is commercially employed as a bleaching, fumigating, sanitizing or sterilizing agent.
Chlorine dioxide can be used to replace chlorine and hypochlorite products more traditionally used in bleaching, sanitizing or sterilizing applications with several resulting benefits. Chlorine dioxide is a more powerful sterilizing agent and requires lower dosage levels than chlorine at both low and high pH levels, although it is not particularly stable at high pH levels. Chlorine dioxide produces lower levels of chlorinated organic compounds than chlorine when sterilizing raw water. Additionally, chlorine dioxide is less corrosive than chlorine to metals and many polymers.
A disadvantage of the prior electrolytic processes used to produce chlorine dioxide is the fact that the chlorine dioxide is produced in the anode compartment of the cell. This requires that the chlorine dioxide be recovered from the anolyte by stripping with air or some other appropriate means to obtain a suitably high conversion rate of chlorite to chlorine dioxide in the electrolyte of typically greater than 20% to make the direct use of the anolyte economically feasible. Attempts to obtain higher conversion rates by operating these electrolytic processes under conditions where more current and lower electrolyte feed rates are employed results in the formation of chlorate and/or free chlorine. Since chlorine is an undesirable contaminant and the formation of chlorate is irreversible, there has been developed a process by which chlorite can be converted to chlorine dioxide efficiently without a separation step.
The use of air stripping as one step in a multi-step purification process to obtain chlorine-free chlorine dioxide is also done in conjunction with the reabsorbing of chlorine dioxide from a generating solution to a receiving solution. This type of a process, however, can be hazardous if the chlorine dioxide concentrations in the air become high enough to initiate spontaneous combustion. Use of a gas-permeable hydrophobic membrane in a purification process has also been developed, but requires costly additional equipment.
Attempts to further significantly enhance the chlorite to chlorine dioxide conversion rate has focused on improved high surface area electrodes, especially anodes, for use in a direct continuous electrochemical process. Concurrently, there has been an increasing need for high selectivity electrodes to achieve higher electrochemical process efficiencies. One of the major technological challenges in producing such an electrode is the efficient coating of the high surface area electrode structure with electroactive metals and/or oxides while using economical amounts of the electroactive coating materials in the electrode structure. This is especially important when the specific surface area of the electrode is very high, such as greater than about 10-25 cm.sup.2 /cm.sup.3, and the electrocatalyst applied to the electrode surface is expensive.
There are many well known and established techniques and methods for the electrodeposition or electroplating of precious metals onto various metallic substrates, using DC current as the driving force for the plating. However, it is difficult to plate or coat structures that are three dimensional and have significant depth, such as metallic felts composed of fibers, ribbons, or woven structures which, when fabricated into electrodes, present high specific surface areas of greater than about 10 to about 25 cm.sup.2 /cm.sup.3.
Electroplating also suffers from the disadvantage of having very limited throwing power, that is the ability of a plating bath to produce deposits of more or less uniform thickness on a substrate structure that has irregularities over the entire surface of the structure. This is especially true on the microscopic scale. Microscopic peaks and valleys on the electrode surface, on the order of microns or less, are not plated evenly because surface projections coat more readily than depressions or crevices. This is due to the difference in plating potentials caused by distance, bulk film layer effects, and concentration/polarization effects. Methods to decrease these effects using chemical agents or current control have been developed, but they still cannot match the plating uniformity of the electroless plating methods.
Electroplating is one technique that could be successfully applied to plate spools of continuous length individual conductive fibers or conductive tow fibers in a reel to reel type of process through a series of baths using properly placed electrodes. However, this process would be difficult to use for plating short or variable length fibers that are not on a continuous spool, such as the melt spun metallic fibers employed in the contemplated electrode structure.
There are also secondary problems caused by hydrogen embrittlement of the fiber structure and coating when using electroplating. Hydrogen is a by-product in the electroplating process that can become incorporated into the coating and substrate. Where titanium is employed, a brittle titanium hydride compound is formed on the electrode surface that will flake off of the surface. If produced in sufficiently large quantities, the titanium hydride formation will destabilize the plated coating by forming voids under the coating, followed by eventual loss by flaking off. This occurs when direct electroplated platinum coatings are made in dilute chloroplatinic solutions. There are dark gray deposits that are found in the solution after electroplating that also rub off of the plated fibers upon handling. The loose deposits, when analyzed by x-ray fluorescence, are shown to contain platinum and titanium.
Another alternative electroplating method to obtain a precious metal plating distribution in a three dimensional felt material can mount the felt against a cathode conductor plate and pass the plating solution through the felt structure. If current is applied at low current density levels, a better plating distribution on the fibers in the felt can be obtained. However, a major disadvantage of this method is that the current levels must be so low (about 1-30 ma/cm.sup.2) to get an even plating that the process may take many hours to complete. Also a significant portion of the plated metal builds up on the conductor backplate. In this method especially, the use of excess precious metal in the plating process is costly.
Electroless deposition techniques have several advantages over electroplating. More uniform deposits with no excessive buildup on projections or edges are obtained. Electroless deposits can display unique chemical, mechanical and magnetic properties and are often less porous than electroplated deposits.
Electroless precious metal plating methods are well known. The most common types plate electroless nickel, copper, gold and palladium. But electroless plating methods for platinum on metals are especially rare because of the plating properties of platinum and the substrates onto which the precious metals are deposited. Achieving a high rate of electrocatalytic performance in view of the percentage of the surface area covered by the platinum or other precious metal plating of a high surface area electrode is a continuing problem. Another concern is the cost of the quantity of precious metal used to complete the coating.
Further, the physical and chemical characteristics of the electrocatalyst coatings and their placement on the electrode surface affect the performance of high surface area electrodes. These include features such as the size and crystallinity of the electrocatalyst material. The chemical and mechanical stability of the electrocatalyst material on the particular electrode substrate is also important and is often affected by the chemical characteristics of the solution environment of the electrochemical process. Oxidation reactions in strong, hot acidic solutions are particularly aggressive environments that affect the effective lifespan of coatings. High current densities also are known to shorten the effective life of an electrocatalyst coating.
Lastly, the substrate onto which the electrocatalyst is deposited can present special coating problems, such as the formation of a stable protective oxide film on the surface that must be removed prior to coating with the electrocatalyst. This is especially true with stable conductive metallic substrates, such as the valve metals of titanium, niobium, zirconium or tantalum that form these oxide films.
These and other problems are solved in the present invention by the improved process to produce an improved electrode and by employing an improved porous flow-through anode structure made by the process in an electrolytic cell using a continuous electrochemical process to produce a chlorine-free chlorine dioxide from dilute alkali metal chlorite solutions in a single step.